Although necessary to recover myocardial function, reperfusion of infarcted tissue is a cause of additional injury which should be therapeutically controlled. Unfortunately, at present, the unique effective post conditioning maneuver is still impracticable in most of the patients and reperfusion-induced injury, after coronary artery occlusion and myocardial ischemia (Ischemia/reperfusion, I/R), results in increased infarct size and development of arrhythmia and contractile dysfunction [1].
Sterile inflammation is a major component of reperfusion injury. Tissue-released danger-associated molecular patterns (DAMPs) that activate cellular receptors triggering inflammasome formation [2] massive release of reactive oxygen species (ROS) and reactive nitrogen species (RNS) [3], neutrophilic infiltrates releasing damaging proteases are among the causes which concur to the progression of cardiac injury upon reoxygenation [4]. Innovative cardioprotective pharmacological interventions should essentially represent an attempt to evoke defensive and anti-apoptotic signals. At this aim, reperfusion conditioning with β-blockers, angiotensin-converting enzyme inhibitors and angiotensin II receptor antagonists, statins and antiplatelet drugs [5-8], as well as antidiabetics [9] and anti-inflammatory agents [10], have been lately applied with mixed results. However, thus far, pharmacological conditioning approaches have fallen short of equaling the efficiency of ischemic postconditioning by intermittent reperfusion/reocclusion cycles. A landmark in postischemic recovery research would be the identification of targets for pharmacological postconditioning among key activators of the noxious cascade inducing myocardial injury. This should specifically occur in the reperfused perinecrotic tissue, named area at risk.
The hypoxic or anoxic insult of the infarcted tissue typically leads to a reduction of metabolic oxidative activity in the interested area, and this is currently regarded as an important contribution to energy depletion and to the insurgence of arrhythmias [11]. Post infarct therapeutic approaches should thus not only limit inflammation and oxidative damages, but also stimulate the recovery of a suitable rate of glycolysis and/or beta-oxidation in the affected tissues.
Ceramide, a sphingolipid mediator and central hub of the sphingolipid metabolism, plays a key role as an inhibitor of proliferation and an activator of both inflammatory signaling and apoptosis [12].
Ceramide accumulation typically occurs via three pathways [13]: 1) the hydrolysis of sphingomyelin by sphingomyelinases (SMases), 2) the de novo pathway, which uses serine and palmitoyl-CoA as initial substrates to generate ceramide through the subsequent action of four classes of enzymes: a serine palmitoyl transferases (SPT), a 3-ketodihydrosphingosine reductase, a dihydroceramide synthases, and a dihydroceramide desaturase, and 3) the salvage pathway, whereby complex sphingolipids are cleaved to release the sphingosine backbone, which can be re-acylated to produce ceramide. The contribution of each of these pathways in ceramide accumulation and induced signaling is profoundly different, due to i) the topology of ceramide formation and interaction with signaling proteins, ii) the amount of ceramide formed and the presence of different ceramide-consuming enzymes such as sphingomyelin synthases or ceramidases, or glycosyl ceramide transferases. Thus, depending on the pathway involved, ceramide signaling can be transient or sustained, or can induce a variety of different effects such as differentiation or apoptosis.
External stimuli and activated receptors can unleash sphingomyelinases thereby regulating membrane lipids raft formation and related signaling pathways [14]. The de novo synthesis, occurring under cell stress conditions, also promotes secondary sphingomyelinase activation [15], which may further amplify stress signals and a ungoverned inflammatory response, suggesting that ceramide accumulates in a self-amplifying mechanism when its de novo synthesis is enhanced under pathological conditions.
Ceramide degradation releases the sphingolipid backbone, which can be further degraded or phosphorylated to form sphingosine-1-phosphate (SIP). SIP antagonizes ceramide, as it exerts pro-survival and antiinflammatory actions. Ceramide and SIP have been described as rheostats, as variations of their relative concentration can drive cell fate in opposite directions [48]. SIP signaling was proved to reduce I/R injury, linking myocardial damage to sphingolipid metabolism [16]. A number of evidences suggest that ceramide is involved in the mechanism of myocardial injury. Myocardial activities of sphingolipid metabolism enzymes were found to be modulated after I/R [17]. Reperfusion after ischemia induces ceramide accumulation in the myocardium [17]. while lowering sphingomyelin [18]. On the other hand, the ischemic preconditioning cardioprotection significantly reduces subsequent reperfusion-induced ceramide increase [19]. Noteworthy, in a recent study on a porcine model of I/R, the separate analysis of the infarct area, characterized by irreversible injury, from the reversibly injured area at risk, detected a selective ceramide increase in the latter [20]. Neutral sphingomyelinase activation was reported in infarcted myocardium [21]. Furthermore, a study on cardiac biopsies from the right auricle treated in vitro showed that inhibition of acid sphingomyelinase activity by desipramine, which indirectly inhibits acid sphingomyelinase activity [22], protected from apoptosis after cardioplegia and reperfusion [23]. In vivo reperfusion with desipramine also conferred partial cardioprotection against I/R [19]. In a model of intestinal I/R injury, ceramide synthesis inhibition by fumonisin B1 reduced oxidative stress and inflammation damage [24].
Desipramine has a broad spectrum of action being a tricyclic antidepressant drug and cannot be considered a specific inhibitor of merely ceramide release [25], fumonisin B1 has a known high toxicity [26] and these considerations limit the application of both drugs.
De novo synthesis of ceramide was implicated in cardiac toxicity and both genetic and pharmacological inhibition of serine palmitoyl transferase (SPT), the first and rate-limiting enzyme in the biosynthesis pathway, improved systolic function and prolonged survival in a murine model of cardiomyopathy [27].
Myriocin ((2S,3R,4R,6E)-2-Amino-3,4-dihydroxy-2-(hydroxymethyl)-14-oxo-6-eicosenoic acid) is a specific SPT inhibitor [28].
It was previously demonstrated that nanocarriers (solid lipid nanocarriers, SLN) loaded with the ceramide synthesis inhibitor myriocin are effective in reducing apoptosis in two different models of proteinophaty-induced oxidative stress and degeneration: retinal photoreceptor loss in Retinitis Pigmentosa [29] and chronic pulmonary inflammation in Cystic Fibrosis [30]. It is also known that the administration of myriocin may be beneficial to heart and circulation conditions associated to glucose metabolism defects of diabetes [31-33].